Parallel-crank type engines have been proposed where two connecting rods are connected to a piston and to respective crankshafts disposed in parallel to each other so that output of the engine can be taken out from the two crankshafts. Devices for taking out engine output from such two crankshafts have been known, such as one where crank gears mounted on the two crankshafts are intermeshed so as to take out engine output from one of the crank gears (e.g., U.S. Pat. No. 5,682,844 which will hereinafter be referred to as Patent Literature 1) and one where engine output is taken out from the two crankshafts via a plurality of gears (e.g., U.S. Patent Application Publication No. 2005/0274332 A1 which will hereinafter be referred to as Patent Literature 2).
FIG. 8 is a partly-sectional side view of the engine output takeout device disclosed in Patent Literature 1. This engine output takeout device 200 includes gears 203 and 204 mounted on two crankshafts 201 and 202, respectively, and a shaft 205 connected to one of the gears 203 to take engine output outside the engine output takeout device. The crankshafts 201 and 202 are connected to a piston 211 via respective connecting rods 207 and 208.
FIG. 9 is a partly-sectional side view of the engine output takeout device disclosed in Patent Literature 2. This engine output takeout device 220 includes: an inner gear 222 mounted on one of crankshafts 221; a ring-shaped output gear 224 having inner teeth 223 meshing the inner gear 222; an output shaft 225 having the output gear 224 mounted thereon; and a gear 228 mounted on the other crankshaft 226 and meshing with outer teeth 227 of the output gear 224.
FIG. 10 is a view explanatory of behavior of the engine output takeout device 200 of FIG. 8, where (a), (c) and (e) schematically show the device 200. (a) of FIG. 10 shows a state in an engine expansion stroke where a lower surface 203b of a tooth 203a of the gear 203, to which the shaft 205 (see (a)) is connected to takeout engine output, contacts an upper surface 204b of a tooth 204a of the gear 204 as shown in (b) of FIG. 10. This is because the gear 203 has a greater moment of inertia than the gear 204 due to a connection with the outside for taking out engine output and thus is more difficult to rotate than the gear 204; in other words, the gear 204 functions as a driving gear, while the gear 203 functions as a driven gear.
(c) of FIG. 10 shows a state in an engine compression stroke where an upper surface 203d of a tooth 203c of the gear 203 contacts a lower surface 204c of a tooth 204a of the gear 204 as shown in (d) of FIG. 10. This is because the gear 203 has a greater moment of inertia than the gear 204 and thus is more difficult to stop rotating than the gear 204; in other words, the gear 203 functions as a driving gear, while the gear 204 functions as a driven gear.
Namely, the gears 203 and 204 alternately function as the driving and driven gears during operation of the engine, and thus, the piston 211 connected to the gears 203 and 204 via the connecting rods 207 and 208 would incline within a cylinder, as shown in (e) of FIG. 10, due to a gap or backlash between the tooth surfaces of the teeth 203a, 203b and the tooth 204a, i.e. a difference in rotational angle between the gears 203 and 204 produced by a backlash. Such inclination of the piston 211 would lead to generation of slap sound and abrasive wear of the piston and cylinder liner.
FIG. 11 is a view explanatory of behavior of the engine output takeout device 220 of FIG. 9, where (a) and (d) schematically show the takeout device 200 including connecting rods 231 and 232 and piston 233 in addition to the crankshafts etc. (a) of FIG. 11 shows an engine expansion stroke where an upper surface 223b of an inner tooth 223a of the output gear 224 contacts a lower surface 222b of a tooth 222a of the inner gear 222 as shown in (b) of FIG. 11. This is because the inner gear 222 has a smaller moment of inertia than the output gear 224 and thus is easier to rotate than the output gear 224; in other words, the inner gear 222 functions as a driving gear, while the output gear 224 functions as a driven gear.
Further, an upper surface 228b of a tooth 228a of the gear 228 contacts a lower surface 227b of an outer tooth 227a of the output gear 224 as shown in (c) of FIG. 11. This is because the gear 228 has a smaller moment of inertia than the output gear 224 and thus is easier to rotate than the output gear 224; in other words, the gear 228 functions as a driving gear, while the output gear 224 functions as a driven gear.
(d) of FIG. 11 shows a state of an engine compression stroke where a lower surface 223b of an inner tooth 223d of the output gear 224 contacts an upper surface 222c of a tooth 222a of the inner gear 222 as shown in (e) of FIG. 11. This is because the output gear 224 has a greater moment of inertia than the inner gear 222 and thus is more difficult to stop rotating than the inner gear 222; in other words, the output gear 224 functions as a driving gear, while the inner gear 222 functions as a driven gear.
Further, a lower surface 228c of a tooth 228a of the gear 228 contacts an upper surface 227d of an outer tooth 227c of the output gear 224 as shown in (f) of FIG. 11. This is because the output gear 224 has a greater moment of inertia than the gear 228 and thus is more difficult to stop rotating than the gear 228; in other words, the output gear 224 functions as a driving gear, while the gear 228 functions as a driven gear.
Namely, during the operation of the engine, as shown in (a)-(f) of FIG. 11, the tooth 222a of the inner gear 222 and tooth 228a of the gear 228 contact the tooth surfaces of the inner teeth 223 and outer teeth 227 of the output gear 224 in the same rotational direction in each of the expansion and compression strokes, and thus, there occurs no rotational angle difference between the inner gear 222 and the gear 228. Namely, the inner gear 222 and gear 228 rotate in constant synchronism with each other, and thus, the piston 233 connected to the crankshafts 221 and 226 via the connecting rods 231 and 232 would not incline.
However, during high-speed rotation and high-load operation or under the influence of torque fluctuation, there is a possibility of the output gear 224 undesirably deforming from a circular shape into a non-circular shape. If different deformations occur at positions of meshing between the inner teeth 223 of the output gear 224 and the inner gear 222 and between the outer teeth 227 of the output gear 224 and the gear 228, the synchronism between the inner gear 222 and the gear 228 would be lost, which results in unwanted inclination of the piston 233.
Further, the tip diameter and pitch diameter of the output gear 224 are determined by the inner gear 223 and gear 228, and thus, when the speed reduction ratio between the inner gear 222 and gear 228 and the output gear 224 is to be changed, there is no other choice but to change the modules of the individual gears, in which case abrasive wear of the tooth surfaces would increase.
Furthermore, because it is difficult to increase the tip diameter and pitch diameter of the output gear 224, the output gear 224 has a small moment of inertia, and thus, the engine output takeout device 220 requires a flywheel in order to reduce rotational fluctuation. As a consequence, the number of necessary components increases, which results in a cost increase. If the diameter of the output gear 224 is increased with the distance between the two crankshafts 221 and 226 increased, the overall size of the engine output takeout device 220 would also increase because the output gear 224 and gear 288 project outwardly beyond the distance between the two crankshafts 221 and 226.